This invention relates to the field of coal beneficiation, and more particularly to an improved flotation process for cleaning of coal and separation of pyrite therefrom.
Naturally occurring coal contains mineral matter impurities such as clays, shales and iron pyrite (FeS.sub.2) which are detrimental to use of coal for the production of clean, efficient energy. When coal is burned for the production of steam, minerals such as clay and shales form ash which builds up and fouls boilers. During combustion, pyrite releases sulfur which is discharged into the atmosphere as sulfur dioxide. Sulfur dioxide is not only a major local pollutant in itself, but may be precipitated as sulfuric or sulfurous acid rain in locations remote from the emission source.
Some of the mineral matter in coal is present in cleat form, or as fairly large particles. These relatively large pieces of mineral matter are readily removed from coal by processes based on the difference in density between the mineral matter and the carbonaceous matter of the coal. Hence, traditional gravity processes such as jigs or shaking tables may be employed. However, some of the mineral matter in coal is present in finely disseminated form. In order to liberate such fine mineral particules, the coal must be ground to a very fine size. When the particle size of coal is reduced to below 28 mesh or 0.7 mm, commonly referred to as "fine coal", gravity processes become ineffective for separation of pyrite and other minerals from the carbonaceous matter of the coal. However, coal whose particle size has been reduced to this point exhibits surface properties that may provide a basis for separation of carbonaceous from mineral matter.
One separation process based on surface properties is froth flotation. In this process, particulate coal is mixed as a slurry with water, and a collector such as kerosene is added in dosages typically of about 0.5-3 pounds per ton of coal. Such a collector makes the surface of the coal more hydrophobic. After the coal slurry containing the collector is conditioned by agitation to assure good phase contact, a frothing agent is added in small amounts to the slurry, producing a flotation pulp which is charged to a flotation cell into which air is also introduced. Air is sparged into the bottom of the cell, producing bubbles which typically range in size from 0.2 to 3 mm. Hydrophobic carbonaceous particles of the coal adhere to the air bubbles and rise with the bubbles to the upper surface of the liquid as a coal laden froth. The mineral matter, which is largely hydrophilic, remains in suspension, and a separation is thus effected.
Although the process of coal flotation is well established for coal fines having an average particle size generally smaller than of 28 mesh, such processes are generally effective only for cleaning of the coal and separation of ash therefrom. Particles of pyrite contained within the coal fines are generally of such small size as to be embedded within the carbonaceous matter, and thus are not effectively separated from 28 mesh or 28.times.100 mesh material. Conditions may be adjusted to concentrate more of the pyrite in the tailings, but this depresses the weight yield and energy value recovery of the carbonaceous matter of the coal. Thus, there is a conflict, or trade off, between energy value recovery and sulfur content of the coal flotation product. To an extent this may be compensated for by using higher dosages of frothing agent, but effective pyrite separation at satisfactory BTU recovery is elusive.
Even the conventional function of cleaning 28.times.100 mesh coal and separation of ash therefrom consumes substantial unit amounts of conventional frothers. Among the frothing agents which are known for coal flotation are pine oil, cresols, isomers of amyl alcohol and other branched C.sub.4 to C.sub.8 alkanols, methylisobutylcarbinol, diisobutyl carbinol 2-ethyl-1-hexanol and polypropylene glycol alkyl or phenyl ethers.
One method for unlocking pyrite enclosed in coal particles is extensive comminution to ultrafine sizes (as fine as 400 mesh). However, this procedure yields a coal product which can be difficult to separate from ash or pyrite by either wet or dry processes, and which may consume particularly high unit volumes of frothing agent if separation by flotation is attempted. Where coal has been comminuted to ultra fine size, the hydrophilic particles may be carried into the froth in layers of water attached to the air bubbles, and the large specific surface of the fine particles generally requires large reagent dosages. Moreover, the influence of surface and electrochemical properties is magnified in utlrafine coal particles, so that the flotation characteristics of fines differ from those of coarser counterparts of the same material.
Efforts have been made in the art to develop processes effective for separation of carbonaceous matter from pyrite and other mineral content of ultrafine coal. Wet processes for such purpose include oil agglomeration, selective flocculation, and coal reverse-pyrite flotation, while dry processes such as high gradient magnetic separation have also been attempted. All these processes have been reportedly successful in reducing pyritic sulfur in coal. However, such processes have not met with a great deal of commercial success because they require extensive retrofitting for implementation in existing plants, and generally have not been proven by full scale process testing.
Because flotation processes have been extensively operated on a commercial scale and there is substantial installed flotation capacity within the coal processing industry, an improved flotation process would potentially provide the most attractive alternative for separation of pyrite and other mineral matter from coal. However, in addition to the problems mentioned above, attempts to process ultrafine coal by flotation are confronted with other difficulties. In a flotation process, the coal particle must first collide with a rising air bubble; then surface interactions must be such that the particle adheres to the air bubble as it rises to the collection zone. Decrease in recovery efficiency with decrease in solid particle size is attributable in part to the reduced probability of bubble/particle collision. The probability that a particle and bubble will collide is adversely affected by factors such as small size, low mass-to-size ratio, surface charges of the particles, and streamlines around the flotation bubble.
Additionally, reagent selectivity, which is based on hydrophobic/hydrophilic interactions between the flotation reagent and the coal surface in an aqueous environment, can significantly influence the overall effectiveness of the flotation process. Adhesion of particles to bubbles is affected not only by these interactions but also by the natural hydrophobicity of the coal particle, induction time requirements, competitive rates of bubble growth, and coalescence and surface tension.
There has been some recognition in the art that bubble size is desirably proportioned to coal particle size. Accordingly, attempts to control the rate of flotation by employing microbubbles have been reported for ultrafine size coal Yoon, "Microbubble Flotation of Fine Coal", Department of Mining and Minerals Engineering, Virginia Polytechnic Institute, Blacksburg, Virginia (1984) reports a study in which introduction of externally generated microbubbles into a column of ultrafine coal yielded improved recovery in product quality as compared to conventional subaeration cells. Jameson et al. (1977) also report work on the concept of microbubble flotation, indicating that advantage may be taken of differences in particle and bubble diameter dependency However, as noted above, as particle size become smaller the specific surface area increases significantly and surface charges and chemical interactions with the reagent become important. It has not been shown that microbubbles in and of themselves can completely control effective flotation of ultrafine coal. Thus, a need has remained for improved processes effective for the flotation of ultrafine coal and particularly for the effective separation of pyrite therefrom, using conventional flotation installations.
Various technologies have been available in the flotation art generally for separation of very fine particles. Thus, for example, Seeton, "Ultraflotation of Minerals and Chemicals", Phillips Corporation Bulletin Number M4-B117, describes a process referred to as "carrier flotation" or "ultraflotation", in which fine particles of the mineral to be recovered are attached to larger particles of a carrier mineral and the resultant aggregates are coated using suitable reagents. This process has been used commercially for the purification of kaolin in which titaniferous impurities are removed from kaolin clay using 60 micron limestone as the carrier particle. However, carrier flotation solves one problem only by creating another, i.e., the need for separation of the desired product from the carrier mineral.
Narasemhan et al, "Column Flotation Improves Graphite Recovery", Engineering and Mining Journal, Volume 84, May, 1972, describes a technique of column flotation which utilizes countercurrent flow to improve separations in a flotation process. In the flotation column, air bubbles rise continuously through a downward flowing slurry, in the course of which bubbles are mineralized and washed free of entrapped gangue particles. The flotation bubbles are smaller than those used in a conventional flotation cell, and column flotation is most effective for particles below 100 mesh.
Dissolved air flotation is widely used for waste water treatment. In accordance with this process, air is dissolved in the water under pressure and, upon release of the pressure, the dissolved air forms a cloud of tiny bubbles which collect and float off impurities. In a similar process, known as microflotation, air is forced through a frit into liquid that has been treated with a surface-tension reducing agent, thus forming tiny bubbles capable of floating particulate matter from waste water. Flocculating ions may also be added so that flocs are formed incorporating into their structure any bacteria or microparticles present. However, dissolved air and microflotation are designed for flotation of all suspended matter, and have not been developed for selective flotation of a desired component.
Because oil/solid interactions due to long range intermolecular forces are much larger than air/solid interactions, it is easier to collect fine particles at an oil/water interface than at an air/water interface. Processes such as oil agglomeration which take advantage of this phenomenon may be termed a form of liquid/liquid extraction. This process has been applied to coal recovery but requires a great deal of oil, for example, from 5-25% by weight on a dry coal basis. Incorporation of air, in which case the process is referred to as emulsion flotation, may help reduce the requisite oil dosage. However, even the latter process requires consumption of a substantial volume of oil, generally rendering the process uneconomical.
In processes known as agglomerate flotation, floto flocculation or aggregative flotation, small particles of a desired mineral are first aggregated and then floated with air bubbles. The larger agglomerates are floated at a rate more rapid than discrete fine particles. Aggregative flotation relies on the selective enlargement of the hydrophobic component particles of the feed material through addition of small quantities of bridging oil (collector). These aggregates are collected by air bubbles in the flotation operation. The collector, typically kerosene, is emulsified with small amounts of oleic acid which may act as a foaming agent. However, the economic feasibility of these processes, including aggregatative flotation, depends on the dosage of bridging oil necessary, as well as the degree to which the oil is able to selectively bridge particles of the desired mineral, as opposed to that from which the desired mineral is to be separated. Turbulence caused by the rise of large air bubbles may result in poor selectivity.
As noted above, efforts have been made in the art to improve the efficiency of coal flotation by use of microbubbles. One process for producing these bubbles is electroflotation in which bubbles 0.02 to 0.1 millimeter in diameter are produced by electrolysis. Such fine bubbles are said to be capable of floating particles below 10 microns in size. While electroflotation is effective for producing microbubbles, it may prove too energy intensive for beneficiation of a relatively low cost resource such as coal.
Meyer et al. U.S. Pat. No. 4,308,133, describes a coal flotation process in which an polycyclic aromatic compound bearing at least one nuclear sulfonic acid or sulfonate moiety is used as a froth promoter to improve recovery of coal. Specifically, the froth promoter is a diphenyl ether which is sulfonated in one or both rings and which may also be alkyl substituted in one or both rings. The promoter is said to be used in a weight ratio of 0.0005 to 0.1 kilograms per metric ton of coal, the working examples illustrating a dosage range of between about 0.006 and 0.016 kilograms per metric ton. The frothing agent is a conventional frother such as polypropylene glycol, methylisobutylcarbinol, diisobutylcarbinol, mixtures of polypropylene glycol and diisobutylcarbinol, 2-ethyl-1-hexanol, etc.
Klimpel and Hansen, "Chemistry of Fine Coal Flotation" describe various flotation processes and reagents used in coal flotation. In the category of dispersants, i.e., compounds which prevent coating of larger coal particles with slimes of clay, etc., this article lists sodium silicates, lignin sulfonates, petroleum sulfonates and polyacrylates.